In printing with a recording apparatus used for inkjet printing, ink is elected from nozzles and then adheres to a recording medium. Since the nozzles are positioned away from a recording medium, printing can be carried out on surfaces having irregular shapes, such as a curved surface and an uneven surface, in a good manner. Hence, such printing has been expected to be widely used in industrial Applications.
In general, inks used in such inkjet recording include aqueous dye inks containing water as the prime solvent and non-aqueous (oil-based) dye inks containing organic solvents as the prime solvent. Typical aqueous dye inks have, however, problems when they are used for industrial purposes, such as an insufficient drying rate of a printed film formed on materials which are less likely to absorb liquid, e.g., plastic films; insufficient adhesive properties (adhesion) of a printed film; and insufficient durability, e.g., abrasion resistance, water resistance, and light fastness. On the other hand, oil-based dye inks contain chromium complex dyes composed of heavy metals such as chromium and therefore have problems in terms of safety.
In order to overcome such problems related to colorants, various inks have been proposed, such as aqueous pigment inks and oil-based pigment inks which contain pigments as colorants, and active-energy-ray-curable inkjet recording inks which are substantially free from non-polymerizable solvents such as organic solvents used for, for example, dilution and dissolution and which can be irradiated with an active energy ray such as ultraviolet light, to cure and dry a printed film.
In the case of using active-energy-ray-curable inkjet recording inks, since a printed film is irradiated with an active energy ray for curing, relatively high durability can be imparted to the printed film. There has been, however, a problem in which adhesion is insufficient in printing on materials which are less likely to absorb liquid, e.g., plastic films. Although a variety of oligomers and adhesive resins may be effectively added to enhance the adhesion, this approach leads to an increase in the viscosity of the ink, which forces printing heads to be driven under limited conditions for stable ink ejection. In particular, in the case where the size of ink droplets is reduced, ejection of small droplets of a highly viscous ink causes problems such as an increase in the number of satellite droplets; a reduction in the precision of landing of ink, e.g., ejection in an unintended direction; and nozzle clogging. Addition of reactive monomers such as tetrahydrofurfuryl acrylate and cyclohexyl acrylate is another effective approach to enhance adhesion; however, such an approach causes problems in terms of VOC, odor, or skin irritation in some cases.
An active-energy-ray-curable inkjet ink composition (for instance, see Patent Literature, 1) has been disclosed as an technique for enhancing adhesion; the ink composition contains polymerizable monofunctional monomers in an amount of 60% to 98% relative to the total amount of polymerizable monomers, one of the monofunctional monomers is isobornyl acrylate, the isobornyl acrylate content is from 75% to 65% relative to the total amount of the polymerizable monomers, one of the monofunctional monomers is N-vinylcaprolactam, and the N-vinylcaprolactam content is from 12.5% to 60% relative to the total amount of the polymerizable monomers.
Use of a light-emitting diode lamp (hereinafter referred to as “LED lamp”) having a low energy has become popular as a light source used for active-energy-ray-curable inkjet recording inks in place of typical light sources such as a metal halide lamp and a high-pressure mercury lamp. A light-emitting diode UV-LED which is an example of LED lamps emits ultraviolet light having the peak emission wavelength ranging from 350 to 420 nm. In the case where the UV-LED is used for inks suitable for traditional light sources such as a metal halide, lamp and a high-pressure mercury lamp, a photopolymerization initiator which can absorb light having a wavelength of around 350 to 420 nm needs to be used; however, since a pigment itself contained in the ink can absorb light having a wavelength ranging from 350 to 420 nm, even use of the photopolymerization initiator which can absorb light having a wavelength of 350 to 420 nm eventually leads to a problem of insufficient curing in many cases. Thus, in an attempt to cure the ink disclosed in Patent Literature 1 with an LED lamp, durability, particularly solvent resistance, cannot be imparted to a printed film in some cases.